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Extension structure

Table of Contents

Many extension-writing guides focus on simple examples first and ignore the requirements of more complex implementations until later. Often such guides must repeat themselves over and over in order to describe these new features. This section describes extension structure from the perspective of a mature, practical implementation, in order to prepare users for needs and issues they will almost always encounter in the process of extension development.

Files which make up an extension

Whether created by hand, using ext_skel, or by an alternate extension generator, such as » CodeGen, all extensions will have at least four files:


UNIX build system configuration (see Talking to the UNIX build system: config.m4)


Windows buildsystem configuration (see Talking to the Windows build system: config.w32)


When building an extension as static module into the PHP binary the build system expects a header file with php_ prepended to the extension name which includes a declaration for a pointer to the extension's module structure. This file usually contains additional macros, prototypes, and globals, just like any header.


Main extension source file. By convention, the name of this file is the extension name, but this is not a requirement. This file contains the module structure declaration, INI entries, management functions, userspace functions, and other requirements of an extension.

The buildsystem files are discussed elsewhere; this section concentrates on the rest. These four files make up the bare minimum for an extension, which may also contain any number of headers, source files, unit tests, and other support files. The list of files in a realistic extension might look like this:

Example#1 Files in an example "real" extension, in no particular order


Non-source files

The .cvsignore file is used for extensions which are checked into one of the PHP CVS repositories (usually » PECL); the one generated by ext_skel contains:


These lines tell CVS to ignore interim files generated by the PHP buildsystem. This is only a convenience, and can be omitted completely without ill effect.

The CREDITS file lists the contributors and/or maintainers of the extension in plain text format. The main purpose of this file is generating the credits information for bundled extensions as used by phpcredits(). By convention the first line of the file should hold the name of the extension, the second a comma separated list of contributors. The contributors are usually ordered by the chronological order of their contributions. In a » PECL package, this information is already maintained in package.xml, for example. This is another file which can be omitted without ill effect.

The package.xml file is specific to » PECL-based extensions; it is a metainformation file which gives details about an extension's dependencies, authors, installation requirements, and other tidbits. In an extension not being hosted in » PECL, this file is extraneous.